Polypropylene provides excellent resistance to high temperature processing, such as at retort conditions of about 250.degree. F. for about 30 minutes. Polypropylene has thus found widespread use, particularly as a sealant layer in pouches which are subjected to heat treatment after the pouch is filled with product and sealed. While polypropylene has been accepted as the material of choice for use in sealant layers of retortable pouches, it has been selected more or less by necessity, as few other materials in the same general price range can provide similar high temperature processability. Likewise, for some uses such as in contact with food or medical solutions, polypropylene is among the few materials which is approved for product contact. Thus, polypropylene provides a combination of desirable properties which is not found in other known materials.
Polypropylene does have associated undesirable properties. Most prominent in packages is the tendency of the polypropylene to be brittle, and to be even more brittle after is has been retort processed. The degree of brittleness of a package film is most easily detected by measuring the capability of a package to tolerate physically stressful treatment. Brittleness of the overall package is a property which is a composite of the brittleness of the individual layers, interfacial bond strengths, the elasticity of adjacent layers and their capability to absorb and dissipate physical shock, and any overwhelming brittleness of any one layer.
The composite brittleness of retort processed packages using polypropylene may generally be observed as the fraction of the packages which fail after being subjected to physical stress or shock.
Polyethylenes in the density range of about 0.91 to about 0.965, and linear low density polyethylenes (LLDPE) have many fine and desirable properties which make them particularly suitable for many uses. Polyethylene ("PE") as used herein is defined as those polyethylenes, homopolymer and copolymer, which have a density in the range of about 0.91 to about 0.965, and LLDPE. Many PE's are stiff and rigid, able to hold shape in thick sections. PE is readily processed and formed in both the solid form and the more fluid plasticated form. It is relatively economical for many uses. In packaging applications it has been found to provide a good barrier to transmission of water vapor. And it is particularly the packaging application which is of interest to the inventors herein, while it will be seen that other applications may abound.
High density polyethylene (HDPE, having a density greater than about 0.94) in particular is known to provide excellent properties of stiffness and resistance to transmission of moisture vapor. It also has acceptable high temperature processing capabilities.
It would be desirable to combine layers of polypropylene and PE into one packaging film to obtain the combined properties of high temperature processability, a strong heat seal, and resistance to water vapor transmission. The package should also have excellent tolerance of physical shock abuse.
Polypropylene and PE layers may be combined into a multiple layer film as by adhesive lamination. Such a construction, though, has undesirable cost disadvantage of the plurality of steps necessary in its fabrication. Further, such films do tend to exhibit the brittleness which is a normal property of polyproyplene. The brittleness may be somewhat improved upon by using a polypropylene copolymer rather than the homopolymer. While there is some improvement in performance with the use of the copolymer, the general level of brittleness is still so high as to be effective to prevent widespread use of such structures.
If, however, an economical structure could be made which would combine the desirable properties of polypropylene and PE while improving on the ability of the package to tolerate physical abuse, such a package may find substantial utility and acceptance. Such a structure may use novel compositions of matter in one or more layers, and might use particular combinations of layers and positional relationships to achieve the balance of properties desired.
It is an object of this invention to provide multiple layer films made from compositions of polymers.
It is a specific object to provide multiple layer films suitable for use in retortable packages which have excellent tolerance to shock abuse both before and after retort processing.
A direct object is to provide closed and sealed packages having increased tolerance to shock abuse.
It is finally an object of the invention to provide novel process for making a multiple layer film which is useful in packaging.